I had exactly the same problem when we moved house and was told by BT that although there was a line at the new property, I needed to pay £124.99 to reconnect / reactivate as it had not been active for 6 months. I told them (before we proceeded to switch) I was going to be shopping elsewhere for a phone provider as I opposed to such an excessive charge, and they agreed to waive £100. Despite taking the names of every representative at BT I spoke to, the full £124.99 was debited from my account. I called them up and they have since credited £100 back. I'm afraid that as you were aware of the charge and proceeded with the activation, you may have problems disputing the charge now.